1. Field of the Invention
The present invention relates to a gap adjusting device of an ink jet printing head for adjusting a gap in which a recording medium passes, and more relates to a gap adjusting device for adjusting the gap between the ink jet printing head and an opposite electrode facing the print head and to a gap adjusting method.
2. Description of the Prior Art
In a conventional ink jet recording apparatus, in a method of applying pressure to ink by a piezoelectric element, a recording method called thermal ink jet (bubble jet) and an electrostatic ink jet printing method are generally known as ink jet recording operations. In these recording methods, it is important to keep a fixed gap (a head gap) between a head and a recording medium.
Specifically, an electrostatic ink jet recording apparatus is different from other ink jet recording apparatuses because an opposite electrode is used in front of the print head via a gap for giving an electric field in the gap. For example, in the electrostatic ink jet recording apparatus as described in WO93/11866 and Japanese published unexamined patent application No. H8-309993 published on Nov. 26, 1996, an ejecting electrode is provided to the print head and a recording paper is arranged between the opposite electrode and the ejecting electrode.
Further, in the electrostatic ink jet recording apparatus, electrified toner particles exist in the ink, and they are distributed in an insulating ink solvent. When a high-voltage pulse with the same polarity as the toner particles is applied to the ejecting electrode, the toner particles are ejected as an ink dot toward the opposite electrode which is separated by fixed distance (gap) from the ejecting part of the print head. The toner particles are ejected when an electric field strength between the ejecting electrode and the opposite electrode exceeds a threshold value. The electric field strength between the ejecting electrode and the recording medium is approximately expressed by the following expression (1): EQU E1=.DELTA.V1/d1. (1)
E1: Electric field strength between the ejecting electrode and the opposite electrode PA1 .DELTA.V1: Potential difference between the ejecting electrode and the recording medium PA1 d1: Distance between the ejecting electrode and recording medium PA1 S: Area of the electrode part between the ejecting electrode and the opposite electrode
If the thickness of the recording medium is d2, and a recording distance between the print head and the recording medium is d1, the following equation is established: EQU .DELTA.V1.epsilon.1S/d1=.DELTA.V2.epsilon.2S/d2 (2-1) EQU .DELTA.V=.DELTA.V1+.DELTA.V2. (2--2)
.DELTA.V1: Potential difference in distance of d1
.DELTA.V2: Potential difference in distance of d2
.DELTA.V: Potential difference in distance of d1+d2 (the gap between the ejecting electrode and the opposite electrode)
.epsilon.1: Permittivity of air
.epsilon.2: Permittivity of recording medium
Therefore, the electric field strength E1 between the ejecting electrode and the recording medium is inserted is established by the following expression: EQU E1=.DELTA.V1/d1 EQU =.DELTA.V/{d1+(.epsilon.1d2/.epsilon.2)}. (3)
E1: Electric field strength in distance of d1
In the conventional type electrostatic ink jet recording apparatus, if the thickness and the permittivity of the paper are different, the recording gap d1 between the ejecting electrode and the recording electrode changes. Hereby, electric field strength E1 between the ejecting electrode and the recording medium is changed and ink ejecting operation on the recording medium becomes unstable due to the change.
A gap adjusting device for detecting and correcting the distance between the surface of paper and a print head is disclosed in Japanese published unexamined patent application No. H4-247975 published on Sep. 3, 1992. The above gap adjusting device mechanically keeps the distance between the surface of paper and the end of a print head fixed; however, the mechanical adjustment is executed manually, and it is difficult to adjust the gap exactly.
Another gap adjusting device for detecting and correcting the distance between the surface of a paper and a print head is disclosed in Japanese published unexamined patent application No. H2-217278 published on Aug. 30, 1990. In the gap adjusting device, an optical sensor detects a gap between the end of a print head and the surface of the recording paper. A gap control device adjusts the gap based upon the detected result from the optical sensor and controls the print head so that an optimum gap is always held.
However, as the gap adjusting device detects only a gap between the surface of paper and the end of a print head, it cannot solve the change of electric field strength caused by the above change of the thickness and the permittivity of the paper. Therefore, the gap adjusting device described in the Japanese published unexamined patent application No. H2-217278 cannot be applied to an electrostatic ink jet recording apparatus.
Further, a gap adjusting device for detecting the thickness of paper and controlling a gap between the surface of paper and the end of a head is also disclosed in Japanese published unexamined utility model application No. H2-56633 published on Apr. 24, 1990. The gap adjusting device detects the thickness of paper and adjusts a gap between a print head and an opposite electrode based upon the detected result.
However, this gap adjusting device does not measure a gap between an ejecting electrode and the opposite electrode and does not correct the change of electric field strength E1 caused by the variation of the thickness and permittivity of the paper. Moreover, a specific mechanism for detecting the thickness of paper is required, and the mechanism is complicated and becomes expensive.
The whole conventional gap adjusting devices have a problem that they are not effective enough to prevent ink ejecting force from changing in a recording apparatus utilizing the electrostatic ink jet recording method.
As described above, in the electrostatic ink jet recording method, electric field strength between an ejecting electrode and the recording medium is an important parameter to eject toner and for stable ejecting, the electric field strength is required to be kept stable independent of the thickness of recording paper.